Microstructure and Performance Comparison of Weld Overlay Cladding Layers on Interchange Rolls After Annealing Treatment

1. Definition and Technical Principles

1.1 Scope of the Technology

This technical entry addresses the post-weld heat treatment—specifically annealing—of weld overlay cladding layers deposited on interchange rolls (also known as clamping rolls or press rolls) fabricated from different substrate materials. The study compares the microstructural evolution and mechanical performance of cladding layers subjected to annealing across various substrate configurations, providing critical data for process qualification and product specification.

1.2 Fundamental Principles

Weld overlay cladding on interchange rolls introduces significant residual stresses, microstructural gradients, and potential defects including cracks, porosity, and unmelted zones. The interfacial region between the substrate and overlay layer experiences complex thermal cycling that can produce brittle phases, columnar dendritic structures, and high dislocation densities. Annealing treatment serves the following metallurgical functions:

1.3 Metallurgical Basis for Cross-Material Comparison

Interchange rolls are commonly fabricated from carbon steel (e.g., 45#, 50#), low-alloy steel (e.g., 42CrMo, 40CrNiMo), or cast iron substrates. The weld overlay cladding layer—typically composed of Cr-Mo-V hardfacing (e.g., D2, H13), Ni-based alloy (e.g., Stellite 6), or austenitic stainless steel (e.g., 309L/316L)—interacts differently with each substrate during both welding and subsequent annealing. The differential thermal conductivity, coefficient of thermal expansion, and phase transformation behavior between substrate types necessitate material-specific annealing parameters and create distinct microstructural outcomes that must be systematically evaluated.

2. Technical Purpose and Strategic Value

2.1 Process Qualification and WPS Development

This comparative study directly supports the qualification of Welding Procedure Specifications (WPS) for interchange roll cladding applications. By documenting microstructural and mechanical performance across different substrate-overlay combinations under standardized annealing conditions, the company establishes empirical data that validates or refines existing procedures, enabling confident procedure qualification per relevant codes.

2.2 Product Delivery Assurance

Understanding the precise effects of annealing on cladding layer properties allows the manufacturing team to:

2.3 Customer Value Enhancement

For end users in steel rolling mills, paper machine manufacturing, and continuous casting operations, interchange rolls require predictable performance over extended service intervals. The comparative annealing data enables the company to provide customers with substantiated recommendations for heat treatment schedules, extending roll life, reducing unplanned downtime, and ensuring consistent clamping performance throughout the roll's operational cycle.

3. Key Process Implementation Points

3.1 Substrate Materials Evaluated

Substrate Type Typical Grades Key Characteristics Thermal Conductivity (W/m·K)
Carbon Steel 45#, 50#, Q345B Lower cost, higher weldability, moderate hardenability 45–55
Low-Alloy Steel 42CrMo, 40CrNiMoA Higher strength, greater hardenability, higher HAZ hardness risk 35–45
Cast Iron HT250, QT600 Graphite structure, lower thermal conductivity, cracking-prone 30–40

3.2 Overlay Cladding Alloys Evaluated

Overlay Type Typical Grades Primary Function As-Welded Hardness (HRC)
Cr-Mo-V Hardfacing D2, H13, Cr12MoV Wear resistance, high hardness 55–62
Ni-Based Alloy Stellite 6, Stellite 21, NiCrAlSi Corrosion + wear resistance 40–48
Austenitic Stainless 309L, 316L Transition layer, corrosion barrier 22–30
Fe-Ni-Cr Alloy Colmon 6, Alloy 6 Transition + wear resistance 35–45

3.3 Annealing Treatment Parameters

The annealing process is the critical variable in this comparative study. Parameters must be tailored based on the combined substrate-overlay system:

Parameter Carbon Steel Substrate Low-Alloy Steel Substrate Cast Iron Substrate
Annealing Temperature 550–650°C 600–700°C 500–600°C
Soak Time 2–4 hours 3–5 hours 2–3 hours
Heating Rate ≤50°C/h ≤40°C/h ≤30°C/h
Cooling Method Furnace cool to 300°C, then air cool Furnace cool to 350°C, then air cool Furnace cool (full)
Preheat Temperature 200–300°C 300–400°C 350–450°C

3.4 Microstructural Evaluation Criteria

The comparative study evaluates the following microstructural and mechanical indicators post-annealing:

3.5 Key Findings and Comparative Insights

Based on the comparative study methodology, the following general trends are established:

  1. Carbon Steel Substrates: The overlay layer retains higher hardness post-annealing due to lower substrate carbon content limiting diffusion-driven softening. The HAZ experiences minimal hardness change, and the interface bond quality is generally excellent with no cracking observed at annealing temperatures up to 650°C.
  2. Low-Alloy Steel Substrates: Higher hardenability results in a wider and harder HAZ. Annealing is more critical for stress relief in these substrates. The overlay layer may experience greater carbon diffusion from the substrate, potentially forming a decarburized zone at the interface. Careful temperature control is essential to prevent over-tempering of the hardfacing alloy.
  3. Cast Iron Substrates: The graphite morphology and lower thermal conductivity create non-uniform heating during annealing. The interface region is most susceptible to cracking. Lower annealing temperatures and slower heating/cooling rates are mandatory. The overlay layer properties are relatively stable but the substrate HAZ requires careful monitoring for graphitization or pearlite decomposition.

4. Applicable Standards and Acceptance Criteria

4.1 Welding Procedure and Qualification Standards

4.2 Heat Treatment Standards

4.3 Material and Testing Standards

4.4 Non-Destructive Testing Standards

4.5 Acceptance Criteria for Annealed Cladding Layers

Parameter Acceptance Requirement Test Method
Overlay Hardness Within ±3 HRC of specified value; no zone below minimum specified hardness ASTM E18 (HRH/HRC)
HAZ Hardness Maximum 380 HV (or per specific WPS); no localized hardening exceeding 450 HV ASTM E92 (HV)
Interface Bond 100% metallurgical bonding; no cracks, voids, or unmelted zones Microstructural examination (100x-500x)
Overlay Cracks No transverse or longitudinal cracks exceeding 0.5 mm length MT/PT per ASTM E165/E1659
Porosity No clustered porosity; individual pores ≤1 mm diameter MT/PT; ultrasonic per ASTM E709
Toughness (if required) Charpy CVN ≥ specified value (typically 27 J at -20°C for low-alloy substrates) ASTM E23

5. Common Risks and Control Measures

5.1 Over-Tempering of Hardfacing Overlay

Risk: Excessive annealing temperature or prolonged soak time causes over-tempering of martensitic hardfacing alloys, resulting in unacceptable hardness loss (e.g., D2 alloy dropping from 60 HRC to below 45 HRC).

Controls: Implement thermocouple-controlled furnace profiles with documented temperature traceability. Conduct hardness verification after every heat treatment cycle. Maintain maximum annealing temperature at or below 650°C for Cr-Mo-V hardfacing overlays. For Ni-based overlays (Stellite), limit to 550°C to prevent excessive grain growth and carbide coarsening.

5.2 Interfacial Cracking During Annealing

Risk: Thermal expansion mismatch between substrate and overlay creates interfacial stresses during heating and cooling that can initiate or propagate existing microcracks.

Controls: Limit heating rate to ≤30°C/h for cast iron substrates and ≤40°C/h for low-alloy steels. Use preheating to reduce thermal gradients. Ensure adequate transition layer deposition (e.g., 309L between carbon steel substrate and Ni-based overlay) to buffer thermal expansion differences.

5.3 Carbon Diffusion and Decarburization

Risk: Prolonged exposure at elevated temperatures promotes carbon diffusion from high-carbon overlay into low-carbon substrate, creating a decarburized zone at the interface with reduced hardness and strength.

Controls: Minimize soak time to the shortest duration achieving stress relief objectives. Use protective atmosphere (N₂ or Ar) during annealing to prevent surface decarburization. Monitor interface hardness profiles on trial coupons before full production runs.

5.4 Grain Coarsening in Overlay

Risk: Excessive temperature exposure promotes grain growth in the overlay weld metal, reducing toughness and potentially causing intergranular cracking during service.

Controls: Perform grain size assessment (ASTM E112) on representative samples post-annealing. Maintain grain size ≥ ASTM No. 6 (fine grain). Implement strict temperature and time controls with furnace calibration verification per NIST-traceable standards.

5.5 Distortion and Dimensional Instability

Risk: Differential thermal expansion during annealing of large-diameter interchange rolls can cause ovality, warping, or dimensional changes that exceed machining allowances.

Controls: Use controlled ramp rates and symmetric furnace loading. Support rolls on appropriate fixtures during heat treatment. Measure dimensional stability (diametral accuracy) before and after annealing per customer specification (typically ≤0.1 mm TIR).

6. Application Across Company Technology Routes

6.1 TIG/MIG Weld Overlay Route

The TIG (GTAW) and MIG (GMAW) weld overlay routes are the primary methods for depositing cladding layers on interchange rolls. The annealing study directly informs the following aspects of these routes:

For TIG overlay specifically, the lower heat input and precise arc control produce finer grain structures that respond more uniformly to annealing, typically achieving better hardness uniformity across the overlay cross-section compared to MIG overlay deposits.

6.2 Hydraulic Explosive Bonding Route

While hydraulic explosive bonding is primarily used for permanent metallurgical cladding without melting, the annealing study provides critical reference data for:

6.3 Explosion Welding Route

Explosion welding produces clad plate or roll segments through high-velocity impact bonding. The annealing study contributes to this route in the following ways:

7. Qualification Building and Certification Contributions

7.1 WPS Qualification Support

The comparative annealing study directly contributes to WPS qualification packages required for customer audits and regulatory compliance. The documented microstructural and mechanical data across multiple substrate-overlay combinations establishes the Essential Variables (per ASME Section IX or GB/T 985) that define the procedure's validity range. This includes:

7.2 Quality Management System Integration

The study findings are incorporated into the company's Quality Management System (QMS) per ISO 9001:2015 requirements:

7.3 Industry Certification Readiness

The technical depth of this study positions the company for:

8. Practical Implementation Guidelines

8.1 Pre-Annealing Preparation

  1. Complete all weld overlay deposition passes and verify overlay thickness, geometry, and surface quality per WPS requirements.
  2. Perform initial NDT (MT/PT) on overlay surface to identify and repair any existing surface defects before annealing.
  3. Document as-welded hardness profile for comparison with post-annealing results.
  4. Verify furnace calibration and thermocouple accuracy within ±3°C tolerance.
  5. Prepare appropriate fixtures to support roll geometry and minimize distortion during thermal cycling.

8.2 Annealing Execution

  1. Load rolls into furnace with adequate spacing for uniform heat distribution.
  2. Attach thermocouples to representative locations (overlay surface, substrate interior if accessible, interface region on coupon).
  3. Execute heating ramp per specified rate (typically 30–50°C/h depending on substrate).
  4. Maintain soak at specified temperature for calculated duration (based on roll diameter, typically 1 hour per 25 mm of diameter as a minimum).
  5. Execute controlled cooling per specified profile (furnace cool to transition temperature, then air cool or furnace cool to ambient).
  6. Document complete thermal cycle with time-temperature trace data.

8.3 Post-Annealing Verification

  1. Allow rolls to cool to ambient temperature before handling.
  2. Perform dimensional inspection (diameter, TIR, runout) to verify dimensional stability.
  3. Conduct transverse hardness survey across overlay, interface, and HAZ regions.
  4. Perform NDT (MT/PT) on overlay surface to detect any annealing-induced cracking.
  5. Prepare metallographic samples from representative locations for microstructural examination.
  6. Compile complete test report documenting all results against acceptance criteria.
  7. Issue heat treatment certificate with furnace ID, cycle parameters, and test results.

9. Conclusion

The comparative study of microstructure and performance of weld overlay cladding layers on interchange rolls after annealing treatment represents a foundational technical asset for the company's qualification building and product delivery capabilities. By systematically documenting the metallurgical outcomes of annealing across different substrate-overlay combinations, the company establishes a scientifically grounded process framework that ensures consistent product quality, supports regulatory compliance, and delivers measurable value to customers through extended roll service life and reduced operational risk.

This knowledge base directly enables the company to execute complex multi-route cladding projects—combining TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—where each technology route requires precisely controlled post-processing to achieve the target performance envelope. The annealing study serves as the critical bridge between cladding deposition and final product qualification, ensuring that the metallurgical integrity of the cladding system is fully realized through appropriate thermal processing.